6.4 Tertiary Filtration & Membrane Bioreactors (MBRs)

6.4 Tertiary Filtration & Membrane Bioreactors (MBRs)

Many Alabama permits now carry limits that ordinary secondary treatment cannot meet reliably, such as low total phosphorus, low TSS, or effluent quality suited to ultraviolet (UV) disinfection or reuse. The WPI Wastewater Treatment outlines for every class list tertiary treatment equipment and processes: filtration and media filtration (sand, anthracite, disc filters), and the Class 4 outline (Alabama Grade IV) adds membrane filtration and membrane bioreactors.

Why Tertiary Filtration?

• TSS and turbidity polishing. Filters remove the fine biological floc that escapes secondary clarifiers.
• Phosphorus removal. Much effluent phosphorus is particulate. Chemical precipitation with alum or ferric salts followed by filtration can reach very low total phosphorus.
• Better disinfection. Particles shield bacteria from UV light and chlorine, so lower TSS improves disinfection.
• Reuse. Many reuse programs require filtration before disinfection.

A tertiary filter is a polishing step. If a bulking secondary clarifier sends a heavy load of solids forward, filters blind quickly, backwash constantly and still fail. Fix the secondary process first.

Granular Media Filters

• Dual-media or deep-bed sand filters work like water plant filters (Section 2.4) but receive more biological solids, so filter runs are shorter and backwash with air scour is important.
• Typical loading for wastewater tertiary filters is about 2 to 6 gpm/ft², with some deep-bed designs higher (Section 14.1).
• Continuous-backwash upflow sand filters send feed upward through a sand bed while sand moves slowly downward. An airlift in the center lifts dirty sand to a washer box at the top, where it is scrubbed and returned to the bed and reject water leaves. There is no separate backwash cycle; operators watch sand movement rate, airlift air flow and reject flow.
• Traveling-bridge filters wash one shallow cell at a time while the rest of the filter stays online.

Common problems include mudballs from inadequate backwash, biological growth in the media, breakthrough during solids upsets and short runs caused by algae or chemical floc carryover.

Cloth-Media Disc Filters

Disc filters use vertical discs covered with pile cloth media. Water flows from outside to inside the discs, and solids collect on the cloth. As the cloth loads, the water level rises; at a set level, vacuum backwash shoes rotate across the cloth and pull solids off while the filter keeps producing effluent. Heavy solids settle in the tank floor and are removed by a solids-waste pumping cycle. Disc filters have a small footprint and short backwash cycles, but the cloth must be inspected for tears and cleaned periodically if grease, algae or chemical scale builds up.

Chemical Phosphorus Removal Ahead of Filters

When the permit requires low phosphorus:

1. Feed alum or ferric chloride ahead of a rapid mix and flocculation zone (or into the secondary process).
2. Allow the metal-phosphate floc to grow, often with polymer.
3. Capture the floc in the tertiary filter.

Jar tests set the dose. Overdosing wastes chemical, adds sludge and consumes alkalinity; underdosing leaves soluble phosphorus. Metal salts also lower pH, which matters for nitrification (Section 6.3).

Membrane Bioreactors (MBRs)

An MBR combines activated sludge with submerged microfiltration or ultrafiltration membranes that replace the secondary clarifier. Permeate is drawn through the membranes by suction, so effluent quality no longer depends on sludge settling.

Operating an MBR:

• Air scour bubbles continuously or cyclically up through the membrane modules to shear solids off the surface.
• Relaxation (pausing permeate flow for a short period) or short backpulses let the cake layer loosen.
• Maintenance cleans with a mild chlorine or acid solution are done on a routine schedule, often weekly.
• Recovery cleans soak the membranes in stronger sodium hypochlorite (for organic and biological fouling) and citric acid (for inorganic scale and metals) when permeability drops.
• Operators track transmembrane pressure (TMP) and permeability (flux divided by TMP, temperature-corrected). A steady rise in TMP at constant flux means fouling.
• Because there is no clarifier, the sludge age (SRT) is controlled only by wasting, and solids inventory is high. Foaming, high mixed-liquor viscosity and fats, oils and grease all make membrane fouling worse.

Worked Example: Disc Filter Loading

A disc filter with 600 ft² of submerged cloth area treats 3.0 MGD.

$$\text{Flow} = \frac{3{,}000{,}000\text{ gal/day}}{1{,}440\text{ min/day}} = 2{,}083\text{ gpm}$$ $$\text{Hydraulic loading} = \frac{2{,}083\text{ gpm}}{600\text{ ft}^2} = 3.5\text{ gpm/ft}^2$$

Compare the result with the manufacturer's rated loading, and remember that peak wet-weather flows can be several times the average.